In this blog post, we’ll explore why tempered glass is much stronger than regular glass, the principles behind it, and how it’s used in our daily lives.
Recently, various violent crimes targeting socially vulnerable groups, such as children and women, have become increasingly frequent and are emerging as a social problem.
I see a man over there. He’s completely engrossed in a mobile game. He pays no attention whatsoever to the people getting on and off the bus right in front of him, focusing solely on his phone. When he hears the familiar bus announcement, the man hurriedly stands up, closes the game, and taps his phone against the transit card reader. As he rushes off the bus—which was eager to depart for the next stop as quickly as possible—he accidentally drops his phone onto the hard concrete pavement. Startled, the man quickly picks up his phone. Fortunately, the screen is intact. Sighing in relief, he continues on his way. In the past, the screen would have cracked like breaking ice, but now his phone’s screen is made of tempered glass as strong as a gorilla.
The most well-known characteristic of glass is that it breaks easily. Many people probably have memories from their childhood of breaking a window while playing ball inside or outside their homes. Broken glass shards are very sharp and dangerous. So why does glass break so easily even from a small impact?
Glass is composed of components such as silicon dioxide (SiO₂), sodium oxide, and lime, with silicon dioxide making up the largest proportion. Glass is an amorphous material that lacks a defined crystalline structure. A crystal refers to a structure in which atoms are arranged in a regular pattern; while most solids exhibit this arrangement, glass does not. Furthermore, glass is characterized by being both amorphous and transparent. Thanks to these characteristics, it is used in a wide range of applications, including bottles, eyeglasses, windows, and cell phone screens. Because it is easy to color, it has also been widely used in works of art; in medieval Europe, churches were beautifully decorated using glass of various colors.
One should not assume that glass is a weak solid just because it breaks easily. Quartz, which has a composition similar to glass, is hard enough to have a Mohs hardness of 7. Ironically, the reason glass breaks so easily is precisely because of its high strength. Tiny imperfections that arise during the manufacturing process play a decisive role in its breakage. These imperfections can be caused by foreign particles on the rollers during the production of flat glass, or by sudden temperature differences between the inside and outside of the glass during the production of blown glass.
Glass is strong under compressive stress but weak under tensile stress. Compressive stress is the force generated inside an object when it is compressed from the outside, while tensile stress is the force generated when the object is pulled from the outside. The maximum compressive stress an object can withstand is called compressive strength, and the maximum tensile stress is called tensile strength. In other words, glass has very high compressive strength but relatively low tensile strength. For example, if you throw a baseball at a pane of glass, compressive stress occurs on the side where the ball strikes, while tensile stress is generated on the opposite side due to bending. If this tensile stress concentrates on a microscopic scratch on the glass surface and exceeds the glass’s tensile strength, the glass will shatter.
Tempered glass is a type of glass that addresses this weakness of ordinary glass. As explained earlier, glass is strong under compressive stress but weak under tensile stress. Tempered glass undergoes a special treatment across its entire surface to create compressive stress. This compressive stress helps counteract the tensile stress that occurs on the surface, preventing it from exceeding the glass’s tensile strength. Methods for creating compressive stress on the surface include thermal and chemical processes, and these two are currently the most widely used.
The thermal method for producing tempered glass is easy to understand if you think of the quenching process used in traditional blacksmithing. When iron is heated to a high temperature and then rapidly cooled, its microstructure changes, making it harder. Glass operates on a similar principle. When glass is heated to a temperature above its annealing point and then rapidly cooled with cold air, the surface hardens first while the interior contracts relatively slowly, forming a layer of compressive stress on the surface. Tempered glass manufactured in this way offers high productivity and is widely used in fields where price competitiveness is essential. It can also be easily found in the glass used in public facilities and in heat-resistant glass tableware.
The second method is chemical strengthening, which utilizes ion exchange. When glass is immersed in molten potassium salt at approximately 400°C, the sodium ions within the glass are displaced and replaced by larger potassium ions. Since potassium ions are larger than sodium ions, they create compressive stress on the surface. This method yields more uniform strength than the heat-treatment method and is also advantageous for manufacturing products of various thicknesses and shapes.
The ion-exchange method is the core manufacturing technology behind “Gorilla Glass,” which is widely used in smartphone displays. Developed by the U.S.-based company Corning, “Gorilla Glass” was initially used primarily in industrial, automotive, and aerospace applications, but as the smartphone era took off, its use rapidly expanded to cover glass for mobile devices. While displays made of ordinary glass are prone to breaking or scratching, Gorilla Glass has significantly reduced these issues. Today, it is used in billions of electronic devices—including not only smartphones but also tablets, laptops, and wearable devices—and advancements in tempered glass technology, alongside the growth of the display industry, are making a major contribution to enhancing convenience and safety in our daily lives.
